270 Darton and Diller — Basalt dikes in the Paieozoie. 



a half miles due north of Doe Hill and the other a dike in 

 Silurian limestones the same distance due J^E. of Doe Hill 

 near the road that extends NW. from Palo Alto. 



The nearest known outcrops of other intrusives are the basalt 

 dikes near Wier's Cave, 40 miles east, and those near Brooks' 

 Gap, 5 miles NW. of Harrisonburg, Va., to which Mr. H. R. 

 Geiger has called my attention. The nearest trap areas in the 

 Trias, are 70 miles east. 



Petrographic notes by J. S. Diller. 



Four specimens collected by Mr. Darton at the localities 

 mentioned above have been examined microscopically. One is 

 of the contact breccia and the other three are of the dikes. 

 All are of the noncrystalline porphyritic type of basalt and 

 differ but little among themselves. The one from the west 

 slope of Jack Mt. on the road to Doe Hill is the best type. 

 The porphyritic crystals of olivine and augite in about equal 

 numbers vary from 1 to 5 millimeters in diameter, and are suffi- 

 ciently abundant to render the rock conspicuously porphyritic. 

 The dense dark groundmass under the microscope is seen to be 

 composed of plagioclase, augite and magnetite. 



The porphyritic crystals of augite are of orange gray color, 

 often with deeper colored brownish borders. The crystal faces 

 are usually rough and jagged. In the groundmass the augite 

 is irregular granular and that of the second generation is fre- 

 quently clamped between the small feldspars. The olivine 

 appears chiefly in porphyritic grains whose outlines suggest 

 crystalline form. Small grains in the groundmass are less 

 common. Like the augite it includes some magnetite, and 

 furthermore a few crystals of a brown octahedral mineral 

 supposed to be picotite. The plagioclase is scarcely as abun- 

 dant as the augite. It is of a basic variety and the lath-shaped 

 crystals are usually smaller than 'OlX'05 millimeters. Magne- 

 tite in crystals and irregular grains is well represented both in 

 the groundmass and as inclusions in other minerals, especially 

 in the augite and olivine. An interesting feature is the occa- 

 sional presence of small scales of biotite in the groundmass, in 

 which respect it resembles the eruptives of the Triassic areas 

 along the Atlantic slope. 



The basalt from the hill a mile SSW. from Monterey 

 differs slightly from the one already noted, being less porphy- 

 ritic. No mica was discovered in its groundmass. The 'frag- 

 ment from the dike of the knoll two miles WSW. of Doe's 

 Hill looks in the hand specimen very like the one from the 

 west slope of Jack Mt., but in thin section it is conspicuously 

 different. Granular augite in the groundmass is much less 

 distinct and opaque black grains became very abundant ren- 



